Structural monitoring system using fiber optics

ABSTRACT

An optical fiber is securely and continuously engaged by a structure such as a pipeline, offshore platform, bridge, building, dam or even a natural object or fluid medium. A light signal is passed into one end of the optical fiber. Any physical movement of the structure, or sectional movements along the optical fiber path, such as deflection, bending, displacement (changes in linear uniformity) or fracture of the structure caused, for example, by stress, strain, pressure, temperature, etc., will necessarily affect the optical fiber. As a consequence, detectable changes will occur in the &#34;electro-optic signature&#34; (for measurements made at the input end of the optical fiber) or in the light signal transmission (for measurements made at the opposite end of the optical fiber). A preferred optic fiber construction comprises a core and cladding contained within a protective buffer coating, wherein the buffer coating includes a particulate material for engaging the cladding in a manner altering reflective or transmission or other &#34;electro-optic signature&#34; characteristics of the fiber when the buffer coating is subjected to radial forces. The particulate material can be uniformly or nonuniformly distributed about the cladding to provide variable sensitivity.

This is a division of application Ser. No. 323,498, filed Mar. 13, 1989(U.S. Pat. No. 4,927,232) which is a continuation-in-part of copendingU.S. Ser. No. 032,042, filed Mar. 27, 1987, now U.S. Pat. No. 4,812,645,issued Mar. 14, 1989, which is a continuation-in-part of copending U.S.application Ser. No. 06/712,889, filed Mar. 18, 1985, now U.S. Pat. No.4,654,520, issued Mar. 31, 1987, which in turn is a continuation-in-partof copending U.S. application Ser. No. 06/571,364, filed Jan. 16, 1984,now abandoned, which application is a continuations of copending U.S.application Ser. No. 06/295,600, filed Aug. 24, 1981, and now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to structural monitoring and moreparticularly, to a method and apparatus for monitoring man-made ornatural structures using fiber optics. Such monitoring may be for thedetermination of integrity or the thermal or pressure related conditionseffecting such structures.

There are many man-made structures, such as pipelines, offshoreplatforms, buildings, bridges, dams and the like for which structuralintegrity monitoring is important to verify design codes, test loadingsand forewarn potential or actual failures of the structure or partsthereof. For example, it is known to place strain gauges, microphonesfor acoustic emissions, tilt meters using accelerometers and the likealong a structure, such as a marine riser or pipeline to provide signalsindicative of strains or deflections beyond normal limits.

Movements in natural objects, such as earth strata or rock formationsadjacent to an earthquake fault, are more difficult to measure becauseof inhomogeneity. Seismic, tilt, meter or land surveying techniques areconsequently employed to measure acceleration, tilting or displacement,respectively.

While the use of strain gauges or other measurement means periodicallyspaced along a structure will serve to aid in providing the desiredstrain data, only those discrete points on the structure to which thestrain gauge is secured are monitored. There may be other locations onthe structure sufficiently spaced from the point of attachment of thestrain gauge or other sensor which will have an influence thereon andyet experience a physical movement or stress which could be significantto the safety or potential failure of the structures.

In addition, power must be supplied to such prior art sensors and datamust be acquired, all of which involves cable, power and telemetryequipment, plus associated logistic and maintenance support. The costfor such a system can become excessive and reliability can becomeimpaired because of the number of elements involved.

Attempts have been made to overcome this difficulty by the developmentof structural frequency-measurement systems placed at a central point ona structure. The technique employs Fourier analysis to detect modalshifts in frequency resulting from changes in structural integrity, suchas fractures in members or even loss of members. In the case of anoffshore platform, however, the variations in loading on the structure,non-linearity of the foundation (piling) and inconsistency of naturalexcitation have precluded sufficient signal to noise ratio to rendersuch a system feasible for identifying the location of structuralchanges.

In view of the foregoing, there is a need for an improved method andapparatus for monitoring some structures wherein all points along thestructure, or between designated parts of a structure, can be"continuously" and reliably monitored. By such a "continuous"arrangement, location of structural movement could be determined, therebeing no gaps in the monitoring system.

SUMMARY OF THE INVENTION

With the foregoing in mind, the present invention contemplates animproved method and apparatus for monitoring structural integrity orthermal or pressure related conditions or changes wherein the monitoringis accomplished by means of fiber optics.

More particularly, for structural integrity involving deflection,bending or displacement, physical movements are monitored between spacedpoints on a structure by attaching at least one optical fiber to thestructure to extend from at least one of the spaced points to the otherand thereby engage all points along the structure as a continuum betweenthe spaced points.

A light signal is then passed into the optical fiber and changes in thislight signal resulting from the physical movements of the structure arethen detected. The detected changes can be a result of reflections,Rayleigh back-scattering, and the like in a light signal (generallypulsed) as a consequence of a movement of the optical fiber. In thiscase, the detected changes are made at the input end of the opticalfiber, these changes being coupled out be means of an optical coupler.

In another embodiment, the light signal (generally continuous) is passedinto one end of the optical fiber and changes therein are detected atthe other end of the optical fiber. In this case, the detected changesare changes in the transmission characteristics of the light signalthrough the optical fiber, such as attenuation or phase shift.

In other alternative applications of the invention, an elongated portionof the optical fiber may be coupled in continuous intimate contact withvarious types of structures with a view toward monitoring specificphysical parameters, such as stress, strain, pressure, temperature, etc.In some embodiments, the fiber is carried by a physical structure suchas a dam or storage tank or other substantially rigid member to detectpressure gradients or variations acting along the length of thestructure. In some cases, the structure may be a fluid with pressureacting upon the fiber. Alternately, the fiber may be provided with aprotective jacket designed to transmit pressure forces from asurrounding fluid to the fiber, in which case the structure may beviewed as the protective jacket. In other embodiments, the fiber may beprovided with a jacket for transmitting temperature responsive forces tothe fiber, with an outer casing being required in some instances toisolate the fiber from surrounding pressure variations; in these cases,the jacket or casing may be regarded as a structure.

A computerized data bank is employed to facilitate the identification ofthe detected changes and to provide an alarm when these changes exceedpreset levels.

A preferred optical fiber construction comprises a core and claddingencased within a specialized buffer coating designed to protect the coreand cladding and further to induce changes in fiber light signals inresponse to radial forces. In this regard, the buffer coating includes afine particulate or aggregate material which may be uniformly dispersedwithin a buffer carrier such as polymer material. Application of radialforces to the buffer coating causes the particulate to intimately engagethe cladding for purposes of altering the characteristics of atransmitted or reflected light signal. In one form, the fiber withbuffer coating can be utilized as a pressure sensor to monitor pressurechanges on a fluid or solid structure. In another form, a temperatureresponsive jacket can be formed about the buffer coating to applyvariable radial forces as a function of temperature changes. In stillanother form, an alternative jacket construction such as a braidedjacket can be used to apply variable radial forces to the buffer coatingin response to axial strain changes.

Other features and advantages of the present invention will become moreapparent from the following detailed description, taken in conjunctionwith the accompanying drawings which illustrate, by way of example, theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of this invention as well as further features andadvantages thereof will be had by now referring to the accompanyingdrawings in which:

FIG. 1 is a cut-away perspective view of a structure in the form of apipeline together with apparatus illustrated in block form formonitoring the structural integrity of the pipeline in accord with oneembodiment of the present invention;

FIG. 2 schematically illustrates partly in block form another embodimentof the structural monitoring system of the present invention;

FIG. 3 is a schematic block diagram similar to FIG. 2 illustrating amodification of the monitoring technique;

FIG. 4 is a schematic showing partly in block form the manner in whichadditional optical fibers can be used in accord with the presentmonitoring system;

FIG. 5 is a block diagram of a monitoring system in which data is storedin a memory;

FIG. 6 illustrates the use of data obtained by means of the system ofFIG. 5 for monitoring structural integrity in accord with thisinvention;

FIG. 7 is a fragmentary perspective view of a fiber optic cablecontinuously attached to a pipeline similar to the showing of FIG. 1 butwherein greatly increased sensitivity is realized by the manner in whichthe fiber optic cable is constructed;

FIG. 8 is a view of the pipeline and cable of FIG. 7 wherein a physicalmovement such as bending of the pipeline has taken place;

FIG. 9 is a greatly enlarged cross-section of that portion of the cablein FIG. 7 enclosed within the circular arrow 9;

FIG. 10 is a greatly enlarged cross-section of that portion of the cableof FIG. 8 enclosed within the circular arrow 10;

FIG. 11 is a cross-section taken in the direction of the arrows 11--11of FIG. 9;

FIG. 12 is a plot of reflection intensity as a function of time and thusdistance, useful in explaining the monitoring techniques;

FIG. 13 is a somewhat schematic diagram illustrating use of theinvention in monitoring earth movements;

FIG. 14 is a somewhat schematic diagram illustrating use of theinvention in monitoring pressure forces acting against structures withinan oil or gas well;

FIG. 15 is a somewhat schematic diagram illustrating use of theinvention for monitoring pressure changes within a fluid, for example,in the ocean floor;

FIG. 16 is a somewhat schematic diagram illustrating use of theinvention for monitoring fluid pressures within fluid-containingvessels;

FIG. 17 is a somewhat schematic diagram illustrating use of theinvention for monitoring pressure forces acting against a structurehaving one or more fibers embedded therein; and

FIG. 18 is a somewhat schematic diagram illustrating use of theinvention for monitoring the temperature of surrounding space.

FIG. 19 is a fragmented perspective view illustrating an alternativefiber optic cable constructed generally in accordance with FIGS. 9-11,wherein a fixed particulate material is integrated into a protectiveouter buffer coating;

FIG. 20 is a somewhat schematic diagram illustrating the fiber opticcable of FIG. 19 in a pressure monitoring application;

FIG. 21 is a fragmented side elevational view of a fiber optic cableconstructed according to FIG. 19, and further including a temperatureresponsive outer jacket; and

FIG. 22 is a fragmented side elevational view of fiber optic cableconstructed according to FIG. 19, and further including a strainresponsive outer jacket.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring first to FIG. 1, there is shown a portion of a pipeline 10constituting an example of a structure to be monitored. In this respect,and as mentioned heretofore, pipelines have been monitored by securingstrain gauges or other sensors at periodic points along the line todetect undue strains or movements in an effort to anticipate buckling orother types of failure along the pipeline.

In accord with the present invention, the monitoring of physicalmovements of the structure is carried out between spaced pointsindicated in FIG. 1 at P1 and P2 by means of an optical fiber 11surrounded by a protective sheath 12 useful in attaching the opticalfiber to the structure 10. The fiber extends from at least one of thespaced points such as P1 to the other such as P2. It will be understoodthat P1 and P2 are merely provided as convenient reference points andwould normally define the end points of the lengths of the structure tobe monitored.

As indicated by the numeral 13, the optical fiber sheath and thereforethe optical fiber itself, is continuously attached to the structure 10so that any physical movement of the structure will result in physicalmovement of the optical fiber.

Appropriate means are provided for passing a light signal into one endof the optical fiber. In FIG. 1, this means constitutes a laserindicated by the block 14.

Also provided are means for detecting and indicating changes in thelight signal provided by the laser 14. This latter detector meansreceives a reverse travelling wave of the light signal coupled out ofthe system by coupler 15 and constitutes a detector 16 and cooperatingsignal processing electronics indicated by the block 17. The detector 16would normally be a photodetector arrangement. The detected changes inthe signal can be read out as indicated by the block 18. By presettinggiven limits, an alarm 19 can be caused to sound should the detectedchanges exceed the preset limits.

It will be understood that any type of physical movement of the opticalfiber such as slight bending will have an effect upon the reversetravelling wave of the introduced light signal along the optical fiber.Thus, various parameters can be detected, such as back-scattering sites,discontinuities, attenuation, and the like. Changes in these parametersresult from the physical attachment of the optical fiber to thestructure and occur as a consequence of physical movements of thepipeline structure 10.

Techniques for indicating the magnitude and location of such changes inthe physical characteristics of an optical fiber per se are well knownin the art. For example, U.S. Pat. No. 4,243,320 discusses a method fortesting optical fibers in which reflected signals produce self-couplingin a laser used to provide the original signal. This self-couplingchanges the lasing activity. Any change in the lasing activity will bean indication of a change in the reflection parameter. Thus, theposition of a reflected discontinuity in an optical fiber can bedetermined by simply monitoring the lasing activity. It is to beunderstood that the present invention is not directed to the specifictechniques in and of themselves since the same are already known.Rather, the present invention has to do with combining an optical fiberwith a structure, either man-made or natural, to enable structuralintegrity to be monitored by means of the optical fiber andelectro-optic measurements.

Still referring to FIG. 1, there is indicated by the blocks 20 and 21 acommunication link at the spaced points P1 and P2. Thus, the sameoptical fiber used for the monitoring operation can also serve as such acommunications link for transmission of voice and/or data and evensignals for control of the entire system. In other words, since theoptical fiber is already in place, it can additionally serve to transmitdata in the manner of conventional fiber optics used in thecommunications industry on a time-share or phase shift basis.

In FIG. 1, the optical fiber is shown secured along the top of the pipestructure 10. It should be understood that the optical fiber could besecured internally on the undertop wall of the pipe if desired.

Referring now to FIG. 2, there is shown at 22 a structure to which anoptical fiber 23 has been attached to extend between spaced points P3and P4. In this embodiment, light from a laser shown at 24 is passedinto one end of the optical fiber 23 and the detecting means ispositioned to pick up the transmitted light at the other end. Thisdetecting means includes a detector 25 and appropriate signal processingelectronics indicated by the block 26. A read-out or display is shown at27.

In the embodiment of FIG. 2, the changes detected are changes in thetransmission characteristics of the light signal.

In both FIGS. 1 and 2, the light signal provided by the laser blocksintroduced into the optical fiber can be either continuous, pulsed oreven polarized or a combination can be either single or multi mode. Asmentioned, the changes in transmission, reflection, back-scattering,polarization, attenuation, phase shift and scattering loss and the likeof the light signal resulting from physical movement of or pressure onthe optical fiber itself, as a result of the physical movements of thestructure, can be detected. The magnitude and location of the physicalmovement or pressure-causing effect can thus be determined.

The light signal injection technique and optical fiber signature ortransmission measurements are similar to those used in the manufactureand testing of optical fibers or for the assessment of fielddistributions along such optical fibers.

Referring now to FIG. 3 there is illustrated another arrangement formonitoring a structural member using an optical fiber and light signaltransmission characteristics. In this embodiment, there is shown astructure 28 having spaced points defined at P5 and P6. An optical fiber29 is continuously secured to the structure 28 as indicated at 30 topass between the points P5 and P6. In this embodiment, the other end ofthe optical fiber 29 adjacent to the point P6 reverses as at 29' andreturns to a point 29" adjacent to the one point P5 to define a loop. Alight signal is passed into one end of the optical fiber 29 as indicatedby the laser block 24 and changes in the transmission characteristics ofthe light signal are detected at the other end 29" as by a detector 25and cooperating signal processing electronics indicated by the block 26connecting to read-out 27. The blocks 25, 26 and 27 may be the same asthe correspondingly numbered blocks shown in FIG. 2.

In FIG. 3, the first portion of the optical fiber 29 is shown attachedto the structure 28 between the points P5 and P6 while the reverseportion forming the loop is not indicated as attached. The purpose forthis showing is simply to indicate that it is not essential that all ofthe lengths of the optical fiber be attached to the structure but ratheronly that portion or section of the optical fiber along the structure tobe monitored defined between appropriate end points. However, it shouldbe understood that the reversed or looped portion could be secured tothe structure at a spaced location so that different structural portionsof the structure can be monitored by the same optical fiber. In thisrespect, it should be understood that the particular structural path tobe monitored need not be straight but can follow any particular pathsuch as bracing members in towers or platforms which can zig-zag backand forth or other non-linear structures.

It will also be noted that there is a portion of the optical fiber 29free of the structure 28 between the laser 24 and the structure. Thisportion 29 of the optical fiber is shown in FIG. 3 to indicate that thelight source for providing a light signal can be located remotely fromthe structure itself, the light signal being transmitted through theoptical fiber to pass into the optical fiber portion between the firstand second points. Similarly the detecting apparatus can be locatedremotely from the structure itself

The significance of the measurements in all of the embodiments describedthus far is that of change. In other words, the optical fiber signatureas determined by back scattering, reflection and the like such as in theembodiment of FIG. 1, or, the light signal transmission characteristicssuch as in the embodiments of FIGS. 2 and 3, are determined when thestructure is in a quiescent or safe condition. It is the change in suchlight signature or light transmission characteristic that constitutesthe significant measurement. As mentioned, the optical fiber itself cantake any path as long as the movement permitted along that path isrepresentative of movement of the structure to be monitored.

Referring now to FIG. 4, there is shown a further embodiment of thepresent invention which will enable not only the location and magnitudeof a physical movement to be measured but also the direction of suchmovement.

As a specific example, there is shown in FIG. 4 a portion of a pipelinestructure 31 to which four optical fibers indicated by the letters A, B,A' and B' are attached. The optical fibers in the example chosen arespaced along four quadrants on the exterior of the pipe structure 31,the pair of optical fibers, A, A' extending along the diametricallyopposite top and bottom surfaces and the pair of optical fibers B and B'extending along opposite diametrical sides.

With the foregoing arrangement, and considering by way of example, thefirst mentioned optical fibers A and A', light is introduced into thefibers by way of a laser 32 and beam divider providing identical lightbeams passing into couplers 33 and 34. Back-scattering reflectioncharacteristics are coupled out and passed into first and seconddetectors 35 and 36 for the respective optical fibers A and A' fromwhich the signals are then appropriately processed in blocks 37 and 38.The output from the blocks 37 and 38 pass to a comparator 39 wherein anappropriate computation is made to determine the direction of anyphysical movement of the pipe affecting the two optical fibers inquestion. An appropriate read-out 40 will indicate such direction.

More particularly, it will be appreciated that should the pipe 31 bendas a result of losing support under a certain section thereof, theoptical fiber A will experience a bending movement having a given radiusof curvature R1 as indicated in FIG. 4. Similarly, the other opticalfiber A' will experience a bending as a result of the bending of thepipe structure 31 but the radius of curvature of this bending will belarger as indicated at R2. Thus, the optical fiber A at the point R1will experience a compression while the optical fiber A' at the point R2will experience a stretching. These changes result in changes in thelight signal all as described heretofore and the changes in the lightsignals themselves are different in that one will indicate a compressionand the other an extension. Thus, a vector or direction of thedeformation as well as the its magnitude and location can be computed.

Similarly, the direction of lateral movements can be determined by thesecond pair of optical fibers B and B', shown in FIG. 4 by utilizingsimilar light introducing and detection circuits.

Referring now to FIGS. 5 and 6, there is shown a still further techniqueof structural monitoring using optical time domain reflectometry oroptical frequency domain reflectometry (OFDR) in accord with the presentinvention. Considering first FIG. 5, there is shown an optical fiber 41into which a light signal from a laser 42 is passed through couplerlight signal passing down the optical fiber 41 make up the optical fiber"signature" of the light signal, and this "signature" is detected in thedetector 44 while the optical fiber 41 is positioned in a predeterminedgiven path or configuration free of any structure. In FIG. 5 thisparticular path is illustrated as a rectilinear path or straight line.

The output from detector 44 for the light "signature" is passed into thesignal processing electronics block 45 and then stored in a memory bank46. It will now be understood that memory bank 46 includes the "lightsignature" for the light in the optical fiber 41 when this optical fiberfollows a specific path free of any structure.

Referring now to FIG. 6, all of the identical elements described in FIG.5 are reproduced except that there is now added a structure 47 to whichthe optical fiber 41 is secured. It is to be noted, however, that thepositioning of the optical fiber on the structure 47 follows theidentical path as described for the optical fiber free of the structurein FIG. 5. As also mentioned, this predetermined path in the examplechosen for illustrative purposes is a rectilinear line.

The same light source such as the laser 42 described in FIG. 5, and samedetector 44 and signal processing electronics blocks 45 are used in FIG.6 as shown and it will thus be understood that the light "signature"detected by the detector 44 will be identical to the light signaturedetected in FIG. 5 if and only if the structure 47 exhibits nosubsequent physical movements or strains that would cause a movement ofthe optical fiber 41 away from its predetermined path as illustrated inFIG. 5.

The output of the signal processing electronics 45 in FIG. 6 rather thanpassing into a memory passes to a comparator 48 for a comparison withthe data stored in the memory bank 46 as a consequence of measurementsmade with the system of FIG. 5. It will now be appreciated that anyphysical movement of the structure 47 resulting in a shifting of theoptical fiber 41 from its predetermined path or straight line in theexample chosen will give rise to a signal that is different from thatstored in the memory 46. This difference will be detected by thecomparator 48 and can be displayed or printed in the read-out 49.

As mentioned, it will be understood that this comparative measurementsystem will be used for any particular predetermined path for theoptical fiber. In the particular example of FIGS. 5 and 6 wherein thepath is rectilinear, the system is useful for providing immediatemeasurements of any positional deviations and their magnitude of thestructure from a rectilinear configuration.

FIG. 6 illustrates a further feature of the present invention which canbe applied to the other embodiments described. Thus, with reference tothe optical fiber 41 secured to the structure 47, it will be noted thatthere are provided markers 50 which are defined as predetermined typesof discontinuities or interruptions in the optical fiber 41 to provide adistinctive type of reflective signal in the light signatures, known asFresnel reflections, which can be easily detected. These "markers" areindicated as being uniformly spaced along the optical fiber 41 to demarkgiven distances such as L1, L2 and L3. The differences in the markercharacteristics are indicated by the different sized arrows 50.

With the foregoing arrangement, there will be provided definitestandards in the detected reflected light signal corresponding to knowndistances; that is, locations along the optical fiber. These standardscan thus be used to maintain the location determining accuracy of themonitoring equipment.

It will be understood that the comparison measurements described inFIGS. 5 and 6 are for those situations wherein it is desired to detect adeviation of a structure from a given configuration wherein theinformation for the desired configuration has previously been stored.

In the actual monitoring systems of the other embodiments, similarprinciples are involved in that there is always being made a comparisonof output data with output data previously received either recently orat remote periods in time. In other words, and as heretofore mentioned,it is the change in the characteristics which are significant in theoverall monitoring operation. Various alternative monitoring techniquescan be used, including but not limited to polarization andinterferometry methods. In such techniques, a schematic system similarto FIG. 3 would be used, with the detector including means for comparingthe signal output through the fiber with the original signal.

As will be evident from the foregoing description, when a physicalmovement is to be detected, the monitoring equipment of FIG. 1 can beemployed with pulsed light and the change in the light signal that isdetected constitutes an OTDR-type signal. The magnitude of the reflectedsignal is proportional to the strain introduced by the physicalmovement. The location or position of the strain change is determined,in turn, by the time it takes for the light signal to make a round tripfrom its starting point at one end of the fiber to the point of thestrain change resulting from the physical movement of the structure andback to the starting point. In the embodiment described, it is theback-scattering of light resulting from light loss at the point ofphysical movement or strain change that is detected. Suchback-scattering is known as Rayleigh reflections as opposed to theaforementioned Fresnel-type reflections which appear in the form ofspikes. Since the speed of light in the fiber optic is known, thedistance from the one end of the fiber optic cable to the disturbance orlocation of the physical movement of the structure is easily computed. Asimilar embodiment can be used for optical frequency domainreflectometry (OFDR), using appropriate chirping techniques.

The sensitivity of the fiber optic to a physical movement orstrain-related deflection can be greatly increased by making use of aphenomenon known as "microbending". This phenomenon can be brieflyexplained as follows: Essentially, an optical fiber functions as awaveguide. It normally includes a core surrounded by a cladding ofmaterial having a different index of refraction from that of the core.Light is thus internally reflected at the interface of the core andcladding and generally precluded from escaping from the core. The lightis thus essentially propagated or guided down the core.

Imperfections at any point along the cladding will result in a change inits refractive index. This change, in turn, permits some light toescape, representing loss; reflection changes also occur at suchdiscontinuity and these reflections, as mentioned, are termed Rayleighback-scattering.

By subjecting the cladding to aggravation, the aforementioned change inrefractive index can be induced from a point on the surface of thecladding. This disturbance will create the same effect as imperfectionsand result in light loss by accentuating microbending and Rayleighreflections.

U.S. Pat. No. 4,421,979 to Asawa et al discloses the use of microbendtransducers which include mechanical amplifiers to facilitate detectionsof remote structural forces. These transducers are located at discretepoints on a structure. An optical fiber free of the structure is engagedperiodically by the transducers so that reflections are only detectablefrom discrete locations.

In accord with an important feature of the present invention, I haveprovided not only a continuous structural monitoring system but alsogreatly increased its sensitivity by providing an improved optic fiberconstruction which, in effect, provides a superior sensor cable. Thiscable incorporates an integral continuum of microbend elements, therebyamplifying the reflected signal from a physical movement of thestructure at any point along the structure to which the fiber isattached.

All of the foregoing will be better understood by now referring to FIGS.7 through 10.

In FIG. 7 there is shown a pipeline 51 with fiber optic cable 52continuously attached thereto as by cement 53 between spaced points P1and P2. The showing is similar to FIG. 1.

FIG. 8 shows the cable 52 subject to a physical movement such as a bendas a result of movement of a portion of the pipeline 51.

It will be understood that a light signal is passed through the cable 52and changes detected by appropriate equipment such as shown in FIGS. 5and 6.

FIG. 9 shows the improved sensor cable 52 in greater detail wherein thefiber optic essentially constitutes a core 54 surrounded by cladding 55.Core 54 has a refractive index of N1 while cladding 55 has a differentrefractive index N2. A first light signal represented by the lines witharrows in the core of FIG. 9 is substantially retained in the core byinternal reflections from the interface 56 of the core and cladding solong as the index of refraction N2 of the cladding is not changed.

In accord with a feature of the present invention, a sheath 57 of wovenfabric or the like such as Kevlar surrounds the cladding and has acoating 58 of fixed particulate material uniformly spread over itsinterior wall surface in intimate engagement with the outer surface ofthe cladding 55. This particulate material may have a representativegrain size of from 5 to 23 microns. The coating 58 is similar to a sandpaper and as mentioned engages the cladding continuously through out thelength of the cable.

Whenever a disturbance occurs, such as a bending of the pipeline portionas depicted in FIG. 8, a strain in the sheath 57 will induce theparticulate material to distort the homogeneity of at least a portion ofthe cladding 55. In other words, a physical movement between the spacedpoints of the structure sets up a strain change in the sheath, claddingand core at a point corresponding to the location of the physicalmovement. In this respect, it will be understood that the spaced pointsP1 and P2 of FIGS. 1 and 7 could typically be several meters or evenkilometers apart. The portion of the pipeline experiencing a bending orphysical movement, on the other hand might be over only a short distanceat any location between the points. It is over this relatively shortdistance that the distortion of the cladding will cause an enhancedback-scattering type reflection due to loss of light. Thus, because ofthe continuous attachment of the cable, any point between the spacedpoints that is disturbed will experience microbend reflections in theoptical fiber at the same point and thereby more accurately determinethe location of such point and the magnitude of the disturbance.

FIG. 10 shows the cable section of FIG. 9 under the bending conditionshown in FIG. 8. It will be noted that the light rays within the coremay still experience some internal reflection at the interface 56 whileother light rays escape such as indicated at 59. As mentioned, the lossof light is a result of the distortion of the cladding by theparticulate coating 58 caused by the bending of the cable.

In FIGS. 9 and 10 the sheath 57 can be the same as the protective sheath12 referred to in FIG. 1. In this respect, the sheath protects the cablefrom contaminants such as radiation, gases, moisture, etc., and alsoserves as an attachment means to the structure and thus is designed totransmit any physical movement to the optical fiber concurrently with"protecting" the fiber from the surrounding environment.

FIG. 11 shows the sensor cable in cross-section wherein it is clear thatthe particulate coating 58 completely surrounds the cladding 55.

FIG. 12 illustrates a logarithmic plot of the measured reflected signalintensity (ordinate) as a function of time (abscissa). Since the lightsignal travels unit distances in unit time intervals, the time is also afunction of distance. The plot shown depicts a situation where adisturbance has taken place in the structure causing bending of theoptical fiber at a point between the spaced points P1 and P2 of thepipeline.

Utilizing a pulsed light signal, back-scattering or Rayleigh reflectionswill occur at the point of loss of light. The normal attenuation of thereflected signal in an optical fiber with increasing distance is shownat 60. At 61 there is shown a sudden loss of light resulting from astructure caused strain on the fiber as described. The distance of thisloss 61 from the zero point on the plot defined the location of thestructural movement and the amount of loss at 61, the magnitude.

It will be recalled with respect to FIG. 4 that the direction of anystructural movement can be determined by providing further sensor cablesaffixed to the structure. In addition, the rate of movement or"dynamics" of the movement can be determined by using the time responseof the structure in a given direction and noting when that movementreturns to its original position.

Further alternative forms of the invention are illustrated in FIGS.13-18, primarily for purposes of depicting a variety of specificapplications of the invention to particular uses in monitoringstructural response to applied forces caused by stress, strain,pressure, temperature, etc. In each case, the invention provides adistributed sensor in the form of a modified optical fiber having aportion in intimate, continuously force-coupled relation with thestructure in a surrounding and/or impacting environment. This opticalfiber can be adapted as desired to insure sensing of the desiredparameter, for example, strain, pressure or temperature.

More particularly, with reference to FIG. 13, an optical fiber sensorcable 100 which can be constructed according to the description of FIGS.9-12 is suspended vertically into a bore hole 102 formed into the earth104. This sensor cable 100 is normally preloaded within the bore hole102 by a weight 106 at the lower end of the fiber. When mounted in thismanner, OTDR or other monitoring techniques as described previouslyherein can be used to detect bending or pressure-induced microbending ofthe sensor cable 100 at any point or points along its length, whereinsuch bending could be caused by lateral spreading or shifting of theearth 104, as depicted in FIG. 13 by arrows 108. Such lateral motion ofthe earth could also be due to tilting or other shifting which mightoccur, for example, at fault lines. In any case, the positional shiftingof the sensor cable 100 can be monitored in terms of magnitude andlocation using the previously described OTDR techniques.

FIG. 14 illustrates mounting of a sensor cable 110 continuously alongproduction tooling 112 or other structures mounted within the casing 114of an oil or gas well or the like. In this arrangement, the sensor cable110 provides a distributed sensor extending continuously along theproduction tooling or the like for remote structural integritymonitoring of the tooling. In addition, or in lieu of mounting to thetooling 112, the sensor cable 110 or selected portions thereof can besuspended freely within the casing 114, illustrated by arrow 116 in FIG.14. In this case, the cable is subjected to fluid pressures within thewell casing 114, with the fluid within the well casing providing the"structure" to which the fiber is attached continuously as a distributedsensor. Detection of tooling movements and/or pressure effects isachieved, for example, by OTDR monitoring, as previously described. Suchpressure effects may be monitored with improved sensitivity by providingthe sensor cable with a jacket of the type described with respect toFIGS. 9-11, wherein said jacket responds to pressure changes andtransmits a resultant force to the encased fiber to induce microbending.

FIG. 15 depicts still another alternative environment of use of theinvention. More specifically, FIG. 15 illustrates an optical fibersensor cable 120 extending along an ocean floor 122 to monitor fluid(water) pressure at various depths to obtain, for example, a profile ofthe ocean pressure. In this illustration, one end of the sensor cable120 extends to an above-water location at an island 124 to permit remotemonitoring of pressure along the entire distributed length of the sensorcable 120 using, for example, the previously discussed OTDR techniques.Once again, the sensor cable 120 is force-coupled over a continuouslength thereof with a structure in the form of the monitored fluid toprovide a distributed pressure read-out The read-out may be enhanced byuse of a pressure transmitting and microbend-inducing jacket or the likeon the fiber, as described in FIGS. 9-11, in which case the jacketcombines with the monitored fluid to provide the force-coupledstructure.

In FIG. 16, still further pressure monitoring applications areillustrated with an optical sensor cable 130 passed through a vessel 132for containing a fluid such as a gas under pressure, In thisenvironment, the cable 130 can be monitored along the length within thepressure vessel 132 to provide a read-out reflecting fluid pressuretherein. Alternately, or in addition, the cable 130 can extend along thebottom of a liquid storage vessel 134 to provide a pressure readingreflective of the liquid level or quantity within the storage vessel. Ineither case, OTDR techniques are usable to perform remote pressuremonitoring. Once again, a protective coating and jacket on the cable maybe used, as described previously, for enhanced sensitivity.

FIG. 17 illustrates an alternative application of structural monitoringto determine the location and magnitude of forces applied to astructure. In this example, one or more sensor cables 140 are embeddedwithin or otherwise suitably continuously attached to a walkway 142.These cables 140 can be monitored as described previously herein todetect forces applied to the walkway 142 due, for example, to thepresence of an unauthorized intruder 144. The continuous force-coupledmounting of the cables within the structure advantageously permits forcemagnitude and location to be identified. Of course, other types offorces may be monitored by embedding one or more cables into other typesof structures, for examples, aircraft wing and frame components,bearings and seals, etc.

In the foregoing strain and pressure monitoring applications, theoptical fiber cable (including core, cladding, coating, jacket, sheath,etc.) can be designed to be temperature insensitive to or controlled (inattenuation change) by the known thermal response of the structure towhich it is attached.

The invention may also be adapted for use in monitoring other physicalparameters such as temperatures. For example, an optical fiber sensorcable generally of the type previously described may be mounted incontinuously attached relation with structures subjected to temperaturevariations to monitor structural size and/or shape changes in responseto temperature changes. Such applications would include, for example,coupling the cable with critical components in high performance aircraftor spacecraft, or coupling the fiber to various processing equipmentsubjected to temperature variations, particularly in environments notsubjected to appreciable simultaneous pressure variations. As viewed inFIG. 18, an optical fiber cable 150 is encased within a protectivejacket 152 designed to respond to temperature variations and to transmitappropriate forces to the fiber cable. The protective jacket thenbecomes the structure and the cable, as a sensor, is subject to thethermal profile experienced by the structure, similar to that forpressure as outlined by FIGS. 13-15. In some instances, when pressure isalso a factor, the fiber cable 150, of FIG. 18, and jacket 152 may beencased within a protective outer liner or casing 154 of rigid tubing orthe like to isolate the fiber cable from the effects of surroundingpressure, for example, fluid pressure at varying depths within the oceanor within an oil or gas well. Again, OTDR or OFDR or similar monitoringtechniques are utilized to obtain a distributed sensor read-outreflective of the sensed temperature changes in the structuralenvironment along the length of the cable. Also, as for pressure, acable coating and jacket may be used, as described previously, forenhanced sensitivity.

FIGS. 19-22 illustrate a preferred optical fiber sensor cableconstruction which can be adapted for use in the various monitoringapplications described above. More particularly, the basic preferredcable configuration is shown in FIG. 19 with a geometry conforminggenerally to that shown in FIGS. 9-11. The optical cable 200 comprises acore 54 surrounded by a cladding 55 having the appropriate refractiveindeces. The core 54 and cladding 55 are encased in turn within a buffercoating 58' which includes the dispersed particulate material aspreviously described with respect to FIGS. 9-11. FIG. 19 illustrates theparticulate material integrated into or mixed within a carrier, whereinthe composite buffer coating 58' protects the fiber cladding 55 inaddition to causing changes in transmitted or reflected light signals inresponse to radially applied forces. A preferred carrier materialcomprises a polymer such as polyimide selected for high temperatureenvironments without significant degradation.

The optical fiber cable 200 of FIG. 19 can be used in a variety ofapplications for monitoring purposes. As one example, as depicted inFIG. 20, the fiber cable 200 can be used without further protectivejacketing in a fluid or other pressure inducing media 202 or the like asa pressure monitor. Radial forces applied locally to the fiber cable 200act through the buffer coating 58' to urge the particulate materialintimately against the cladding, resulting in light signal changes aspreviously described. The effective continuous coupling between thebuffer coating 58' and the monitored media permits close pressuremonitoring in adverse or otherwise difficult to access environments,such as within oil wells, etc. Moreover, the monitored media 202 maytake other forms, such as a solid having the fiber cable 200 embeddedtherein. By use of the buffer coating 58' which integrally includes theparticulate, the entire cable can be constructed with an extremely smalldiameter compatible, for example, for embedment into aerospace or otherstructures which may be formed from composite materials. The size andquantity of particulate used in the buffer coating can be tailored toselect specific system resolution, with a submicron particulate sizeproviding the desired resolution in some systems. Moreover, theparticulate can be used in variable grain size or in variable densitygradient along the cable length to provide a selected system sensitivitywhich can be designed to avoid frequency dependent alterations.

FIG. 21 depicts the optical fiber cable 200 inclusive of the buffercoating 58' encased within an outer jacket 57' of a temperativeresponsive material. In particular, the outer jacket 57' is formed fromvirtually any jacketing or sheath material such as a selectedelastomeric compound chosen to expand and contract in response totemperative variation. With this construction, the jacket 57' appliesvariable radial forces to the buffer coating 58' as a temperativefunction. These variable radial forces correspondingly alter thespecific engagement between the particulate and the cladding to permitremote temperature monitoring of a fluid or solid structure.

FIG. 22 shows the optical fiber cable 200 jacketed by an alternativeouter jacket 57" formed from a woven fabric of Kevlar or the like. Thejacket 57" is woven in a Chinese braid or the like to apply variableradial compression to the fiber cable 200 in response to movements ofthe fabric jacket. In a typical preferred application, the jacket 57"can be continuously coupled with an appropriate structure to monitorstructural movements induced by strain or the like. Such movementsresult in longitudinal extensions or retraction of the jacket 57" andcorresponding alteration of compressive forces applied to the fiber.Once again, particulate size and quantity and/or distribution can bevaried as needed to provide a desired system resolution.

From all of the foregoing, it will now be evident that the presentinvention has provided an improved method and apparatus for enablingcontinuous monitoring of a structure for integrity as well as forthermal or pressure related conditions or effects and to cover allpoints between starting and end points on the structure, including thosecomprised of fluid or gas mediums. In other words, the output of thissystem output will be continuous relative to the structure, not atdiscrete locations only as with strain gauges previously mentioned orother prior art systems employing microphones, tilt meters usingaccelerometers, discrete microbend transducers, and the like.

Various further applications and modifications falling within the scopeand spirit of this invention will occur to those skilled in the art. Themethod and apparatus accordingly are not to be thought of as limited tothe specific embodiment set forth merely for illustrative purposes.

What is claimed is:
 1. An apparatus for monitoring a physical movementbetween spaced points on a structure including, in combination:at leastone optical fiber; means for coupling said optical fiber continuously tosaid structure to extend from at least one of the spaced points to theother and thereby engage all points between said spaced points so thatphysical movement of the structure at a location between said spacedpoints will result in physical movement of the optical fiber at alocation corresponding with the location of movement of the structure;means for passing a first light signal into said optical fiber; andmeans for detecting and indicating changes in said first light signal tothereby detect that a physical movement has taken place; said at leastone optical fiber comprising an optical fiber sensor cable including acore, a cladding surrounding said core and having an index of refractiondifferent from the index of refraction of said core, and a coatingsurrounding said cladding, said coating including a particulate materialin intimate engagement with said cladding, said particulate materialbeing movable radially toward and away from said cladding to distort thehomogeneity of at least a portion of said cladding to change the indexof refraction thereof.
 2. The apparatus of claim 1 wherein saidparticulate material engages the cladding continuously throughout thelength of said cable.
 3. The apparatus of claim 1 further includingmeans for moving said particulate material toward and away from saidcladding.